Synthetic cord for tennis racket

ABSTRACT

Synthetic cord, specifically for tennis racket. According to the invention, the cord consists of a core, which consists of at least one small polyester monofilament ( 1 ), surrounded by small monofilaments ( 2 ), where a polyester elastomer layer ( 3 ) is enclosed between the various component layers. Applications: tennis cords, for easy cording, featuring a long life span, and a sharp reaction to the impact of a ball.

The subject of the present invention is a synthetic cord, designed specifically, but not exclusively, for cording tennis rackets, or rackets for other similar ball games, such as squash or badminton. The invention also involves a device that gives flexibility to the cord, so as to allow rackets to be corded more easily by improving the life span of said cords with improved play characteristics.

Composite synthetic cords are already well known. FR-A-2 491 098 describes a synthetic cord with two components; multifilament threads of polyamide and polyurethane, where the multifilaments are integrated into a polyurethane matrix. The polyurethane binder, whose elastic behavior is greatly superior to that of polyamide, allows us to achieve, with a specific cord structure, a higher resistance to breakage and a mean hardness that translates into rapid return of the cord to its initial position after the impact of a ball.

Monofilament tennis cording is also known, made of extruded polyester, which features a life span that is superior to that of cords noted above, but with inferior play characteristics. These cords, because of their high rigidity, are very difficult to cord, such that this operation requires greater time that with one of the cords indicated.

The object of the present invention is to considerably lengthen the life span of tennis cording, even those used with rigid-frame rackets, with good play characteristics.

According to the present invention, the synthetic cord for tennis racket or other game racket, consisting of polyester, is distinguished by the fact that it consists of small monofilaments from the polyester family. The cord also consists of polyester threads, structured in the shape of small-diameter monofilament cabling.

The term “polyester family”, shall be understood to mean not only polyester, but also its copolymers, such as polyester-ether-ether copolymer. The monofilament diameter shall be, for example, between 10 and 45 hundredths of a millimeter.

The invention is based on the idea that, although a small-filament cabling leads to a material density inside the cord that is lower than that of a monofilament of the same diameter, it offers greater tenacity through better orientation of the chains of molecules as the low-diameter threads are being pulled, than when the large-diameter monofilaments are pulled. Thus, assembly of small monofilaments offers greater tenacity than that of a monofilament of equivalent diameter, as well as greater cord flexibility. Moreover, the cording provides flexibility to a material that is intrinsically rigid, which is a great advantage when it is used for racket cording.

According to another characteristic of the invention, the threads are cabled according to a metallic cabling diagram. The “filling” of the cord, i.e. its density is, thus, greater than that which is achieved by standard cabling of textile fibers.

But if, in metallic cables, cohesion of threads is assured by the rigidity of these threads, it is desirable, with textile fibers, even rigid ones like polyester, to provide adherence of these fibers, which can be assured by superficial fusion or by superficial dissolving of threads in contact, with the help of a polyester solvent, followed by an autogenous gluing.

According to yet another characteristic of the invention, in order to assure cohesion of the polyester threads that comprise the cord, we introduce, preferably between the threads, a thermoplastic polyester elastomer consisting of a polybutylene teraphtalate base which requires no vulcanization, such as those that are manufactured by the DSM companies under the ARNITEL® trademark, or by DUPONT DE NEMOURS under the HYTREL® trademark.

Small elastomer filaments are coated, then are cabled according to a metallic diagram, and heated so that the elastomer, which is initially liquid, and which may be mixed with a stabilization primer, hardens until it reaches a hardness on an order of 30 to 70 Shore D. Before hardening, which takes place at a temperature on an order of 80° C., it is possible to pass this assembly through a smoothing die, which creates a coating, such that the resulting cord is perfectly cylindrical.

The small-monofilament spools are mounted on a turning platform, on which the threads are coated with a polyester elastomer solution. Then the threads are cabled in a ring and the binder is permanently polymerized in an oven. The cord can thus be made in a single operation without additional passes.

Other characteristics and advantages of the invention will become apparent during the following description of specific manners of embodiment, which are given solely as non-limiting examples, with regard to the drawings, which show:

FIG. 1, a view of a cord with a core and small polyester monofilaments according to the invention;

FIG. 2, a schematic view of an installation that allows us to obtain, in the first step, a cord such as that in FIG. 1;

FIG. 3, a schematic view of the structuring step;

FIG. 4, a partial cutaway view of a cord including a braid;

FIG. 5, a second example of cord obtained with the process according to the invention;

FIGS. 6 and 7, which are yet another example of metallic-type cabling of small polyester monofilaments.

On FIG. 1, we see that the cord according to the invention consists of a core 1, consisting of an assembly of small monofilaments, the whole of which has a diameter of 0.9 mm around which are layered sixteen polyester monofilament threads (2) with a diameter of 0.2 mm.

The intertwining manufacturing process of this type of cord consists of coating the small monofilaments in order to comprise core (1) with a polyester elastomer, before twisting, then to intertwine them with a torsion rate of 45 to 150 turns per meter. After intertwining, core (1) is passed through a contour smoothing die which removes the excess polyester elastomer. Core (1) can also consist of a single monofilament.

FIG. 2 schematically shows an installation that allows us to manufacture a cord such as the one shown in FIG. 1. Core (1) is made by intertwining small monofilaments. It is unrolled from a feeder spool (15) from which it passes over a tension pulley (16), after which core (1) passes through a polyester elastomer dose applicator tank (17). The viscosity of the latter is adjusted so that the core collects only the necessary quantity of elastomer.

Above tank (17) is wrapping point (18), which is practically a ring, and which moreover accepts wrapping threads (2). In FIG. 2, only two outflow spools (19) are shown, whereas in the cord shown in FIG. 1, sixteen spools are mounted on a turning platform (not shown), a spool which unwinds only one thread (2).

Above wrapping point (18) an oven (10) is provided, which heats the cord to a temperature on an order of 80° C. and which hardens the elastomer and glues the threads to the periphery of the core. The resulting cord (12) then passes over a return pulley (13) and onto and over a capstan (20) before being wound onto a receptor spool (14). The cord can then be used as it is.

The same process can be used to apply an additional layer of threads, for example up to thirty-two, but of lesser diameter, which are glued to cord (12).

Nevertheless, it can be useful to give the cord an exterior shape that is perfectly cylindrical by application of an external layer of elastomer, which constitutes a protective layer around the cord in order to improve resistance to abrasion and to indentation caused when the cords rub against each other when the racket is corded. To this end, it is possible to recover the cord with a layer or covering in the second step, during which the furrows created during cabling are filled up with the elastomer.

This operation can be carried out on a device such as that shown in FIG. 3. Spool (14) unwinds thread (12) onto a tension pulley (25). Cord (12) is coated with elastomer in the applicator tank (26) and heated in oven (27). As the cord passes through tank (26), a calibrating head allows us, on the one hand, to smooth and limit the diameter of the coated cord and, on the other hand, to make the elastomer penetrate into the furrows resulting from wrapping or cabling. The sheathed cord (4) thus passes next over a return pulley (29), then over a pulling capstan (30) and is wound onto a receptor spool (31).

The two cord manufacturing operations may be carried out continually on a single machine with the receptor spool (14) replaced with a single capstan.

On FIG. 4, core (1) consists of small intertwined polyester monofilaments. It is surrounded with a braid (2) of small polyester monofilaments and covered outside with a structural wrap (4) of polyester elastomer. An elastomer layer (3) is interposed between core (1) and braid (2).

In FIG. 5, core (1) consists of a single monofilament (1) surrounded by six monofilaments (2 a), which are themselves surrounded by twelve monofilaments. In this example, which is detailed below, all monofilaments are of the same diameter.

EXAMPLE

The cord shown in FIG. 5 is of the 1+6+12 type. Its threads are of identical diameter and regrouped according to a metallic cable diagram. The cord consists of a central thread (1), a primary layer of peripheral threads consisting of six threads (2 a), where threads (2 a) are covered with twelve threads (2 b). The cord is of a diameter of 1.3 mm and has a resistance to breakage that is greater than 50 daN. Its breakage lengthening is on the order of 20%, and its mass is 1.8 grams per meter. It is composed of nineteen polyester threads whose diameter is 0.25 millimeters and which, as they are being cabled, are wound in a spiral, and intertwined at 80 turns per meter.

In FIGS. 6 and 7 another example of cabled cord is shown, according to a metallic cabling of the 1+5+(5+5)−18 type. Around cord (1), which consists, for example, of a monofilament 0.16 mm thick, are cabled five monofilaments (2 b) whose diameter is 0.21 mm. On this layer, five monofilaments (2 c) are cabled, their diameter is 0.24 mm, and between which are interposed five monofilaments (2 d) whose diameter is, for example, 0.18 mm. The whole is covered with eighteen monofilaments (2 e) 0.17 mm thick. The three first layers are cabled in an “S” whereas the outside layer is cabled in a “Z” in order to get an anti-gyrating effect.

In general, it is advantageous to assign opposite cabling directions, between the different monofilament layers shown in FIG. 7, but that is not in any way necessary, since cohesion between the layers is ensured by the elastomer.

The polyester elastomer used in the construction of a cord according to the invention has only one goal, to maintain cohesion of the polyester threads, and does not intervene in the characteristics in play of the cord.

Although, in the preceding description, the small monofilaments are made of polyester, this material can be replaced by any other copolymer having equivalent dynamic characteristics.

It goes without saying that many alternative variations can be bestowed on the invention, notably by substitution of similar means, without leaving the scope of the invention. 

1. Synthetic cord for tennis racket or other game racket, consisting of polyester, and distinguished by the fact that it consists of monofilament polyester threads of low diameter structured by cabling.
 2. Synthetic cord according to claim 1, distinguished by the fact that threads are cabled according to a metallic cabling scheme.
 3. Synthetic cord according to claim 1, distinguished by the fact that the cohesion of the polyester threads which comprise the cord, is ensured by a polyester thermoplastic elastomer (3).
 4. Synthetic cord according to one of the preceding claims, distinguished by the fact that the diameter of the monofilaments is between 10 and 45 hundredths of a millimeter.
 5. Synthetic cord according to one of the previous claims, distinguished by the fact that a structuring coating (4) surrounds monofilaments (2).
 6. Manufacturing device for a cord according to one of the preceding claims, consisting of a rotating platform, distinguished by the fact that the monofilaments pass through a coating tank by a polyester thermoplastic elastomer, then in an oven. 